The present disclosure relates generally to a system for, and a method of, estimating true bearings of radio frequency (RF) identification (RFID) tags associated with items located below an overhead array of antenna elements in a controlled area, especially RFID-tagged items located in a zone directly underneath the array.
Radio frequency (RF) identification (RFID) technology is becoming increasingly important for logistics concerns, material handling and inventory management in retail stores, warehouses, distribution centers, buildings, and like controlled areas. An RFID system typically includes an RFID reader, also known as an RFID interrogator, and preferably a plurality of such readers distributed about the controlled area. Each RFID reader interrogates at least one RFID tag, and preferably many more RFID tags, in its coverage range. Each RFID tag is usually attached to, or associated with, an individual item, or to a package for the item, or to a pallet or container for multiple items. Each RFID tag typically includes an antenna, a power management section, a radio section, and frequently a logic section containing a control microprocessor, a memory, or both. Each RFID reader transmits an RF interrogating signal, and each RFID tag, which senses the interrogating RF signal, responds by transmitting a return RF response signal. The RFID tag either generates the return RF response signal originally, or reflects back a portion of the interrogating RF signal in a process known as backscatter. The return RF response signal may further encode data stored internally in the tag. The return signal is demodulated and decoded into data by each reader, which thereby identifies, counts, or otherwise interacts with the associated item. The decoded data, also known as a payload, can denote a serial number, a price, a date, a destination, other attribute(s), or any combination of attributes, and so on.
The RFID system is often used in an inventory monitoring application. For example, in order to take inventory of RFID-tagged items in a retail store, it is known to position at least one RFID reader overhead in a controlled area, and then, to allow each reader to automatically read whatever tagged items are in the coverage range of each reader. For superior RF coverage, it is known to provide each reader with at least one overhead array of antenna elements that are arranged about a central vertical axis, also known as a plumb line, and that transmit the RF interrogating signal as a primary transmit beam that is electronically steered both in azimuth and in elevation, and that receive the return RF response signal via a primary receive beam from the tags.
As satisfactory as such known RFID systems utilizing antenna arrays have been in monitoring inventory, they can also be used for locationing applications, i.e., for accurately estimating and determining the true bearing, i.e., the angular direction both in azimuth and elevation, of any particular tag, relative to a particular reader. However, there is a practical limit on the number of antenna elements that can be used in each array. This antenna element limit causes each primary transmit beam and each corresponding primary receive beam to have a relatively broad beam width. The primary transmit beam is typically steered until the reader reads the tag with the highest or peak receive signal strength (RSS) of the primary receive beam at a primary steering angle. However, estimating the bearing, i.e., the angular direction both in azimuth and elevation, of any particular tag based on the peak RSS of the primary receive beam is imprecise due to the aforementioned relatively broad beam width. Bearing errors on the order of 5 to 10 degrees have been reported and are not readily tolerable in locationing applications.
To improve the accuracy of estimating the location of a particular tag, it has been proposed in the above-identified patent application to generate multiple secondary receive beams pointing in different directions to independently measure the peak RSS for a particular tag. These secondary receive beams are processed to generate azimuth and elevation error signals as azimuth and elevation corrections to the primary steering angle of the primary receive beam, thereby reducing the bearing error.
Yet, as advantageous as the RFID system disclosed in the above-identified patent application has been in accurately locating the true bearings of tags generally located in the controlled area, there is a so-called “dead” zone in the controlled area in which the true bearings of any tags therein are not so accurately estimated, if at all. This dead zone is the scan area directly underneath the antenna array. More particularly, this dead zone extends over a conical, three-dimensional, spatial sector subtending a solid angle whose vertex is centered on the aforementioned vertical axis or plumb line. Still more particularly, this dead zone extends downwardly away from the vertex and measures about 30 degrees in elevation on each side of, and upwardly away from, the vertical axis, and 360 degrees in azimuth around the vertical axis.
This dead zone is a result of the known practice of energizing circumferentially adjacent antenna elements with the same polarization or orientation of its electric field. For example, it is known to energize two pairs of circumferentially adjacent antenna elements, all with the same vertical (or horizontal) polarization, to obtain four commonly-polarized secondary receive beams. This, however, causes the beams of the energized antenna elements to exhibit low directivity towards the dead zone, and also causes little electromagnetic power density to be present in the dead zone. The amount of such electromagnetic power density in the dead zone may be sufficient for monitoring applications, but is less sufficient, and often insufficient, for locationing applications. The above-described primary and secondary beams are thus not steerable through the dead zone. As a result, the known RFID system may detect a tag in the dead zone, but cannot readily determine or estimate its true bearing.
Accordingly, there is a need to estimate the true bearings of RFID tags located anywhere in a controlled area, and particularly in the zone directly underneath an overhead array of antenna elements of an RFID reader.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.